In recent years, gas fields and oil fields are newly developed in abundance due to an increase in demand for gas and petroleum. There are increasing opportunities to embed pipelines that transport gas and petroleum in earthquake-prone areas and non-permafrost areas. However, in such earthquake-prone areas and non-permafrost areas, the ground may be moved due to various causes such as liquefaction, fault displacements, frost heaving, and thawing, and the pipelines may be deformed accordingly. Further, when a pipeline is deformed significantly, steel pipes structuring the pipeline are bent. The pipe can then buckle on the compression side and subsequently break on the tensile side. With these circumstances in the background, techniques of improving deformability of steel pipes so that the steel pipes bend without buckling have been proposed from a viewpoint of preventing damages in the buckling part and preventing leakage of gas or petroleum from the broken part. More specifically, Japanese Patent No. 5447461 (see claim 1 and paragraph 0044) describes a technique of improving the deformability of a steel pipe by arranging a wavelength ratio (the wavelength of a waveform shape/a Timoshenko's buckling wavelength) of the waveform shape (undulation) to be 0.8 or smaller, the waveform shape being formed by outer diameters along the longitudinal direction of the steel pipe through a pipe expansion process.
According to the technique described in Japanese Patent No. 5447461 (see claim 1 and paragraph 0044), while the amplitude of the waveform shape is arranged to have a constant value throughout (0.73 mm=0.06% of OD, where “OD” denotes the diameter of the steel pipe), the range of the wavelength ratio of the waveform shape that can improve the deformability of the steel pipe is defined. However, as a result of extensive studies, we discovered that, even when the wavelength ratio of the waveform shape is in the abovementioned range, deformability of steel pipes may be lowered in some situations depending on the value of the amplitude of the waveform shape. Further, generally speaking, the smaller the wavelength ratio of a waveform shape is, the shorter is the forwarding pitch of the die in the longitudinal direction of the steel pipe during the pipe expansion process. Therefore, when the technique described Japanese Patent No. 5447461 (see claim 1 and paragraph 0044) is used, the labor and time required by the pipe expansion process increase in accordance with the improvement of the deformability of the steel pipe. For this reason, there is a demand for a technique that is able to improve the deformability of steel pipes while reducing the labor and time required by the pipe expansion process.
In view of the circumstances described above, it could be helpful to provide a steel pipe, a steel pipe structure, a method of manufacturing a steel pipe, and a method of designing a steel pipe with which it is possible to improve the deformability while reducing the labor and time required by the pipe expansion process.